1. While we never see any adults, they're mentioned, and in one episode parts of them are visible on camera.

2. Nothing in the show indicates they are from different parts of the 20th century; the show creators have mentioned they've included a mishmash of technologies and things from different eras so it's relatable to both adults and kids.

3. The fact that Jimmy "died" in the early 2000s in this theory doesn't take into consideration that the show debuted in 1999.

4. One episode shows how the Eds met as young children, and they were not deceased.

The show is simply a more demented version of Peanuts, especially since they both focus on kids and the adults in their lives are mentioned but not seen or heard.